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Small stars may have Habitable Zones, but habitable planets might not be common (fcsuper.blogspot.com)
38 points by fcsuper on Dec 5, 2016 | hide | past | web | favorite | 10 comments

I'm not an astronomer, though I do work with them. My impression from talking to them is that there is no consensus around whether or not habitable planets are common or rare.

Our principal detection method for finding planets excels at finding gas giants with masses similar to their host star. This is not a great tool for finding habitable planets. Direct imaging of planets is not feasible at an intergalactic scale; I'm not sure it's even really possible for stars further out than a few hundred lightyears. This greatly limits our sample size. The less you know about your distribution, the less accurate your simulation is going to be.

A priori arguments from simulations have the problem of either producing an absurd cornucopia of habitable planets or making too few. Steven Dole's _Habitable Planets for Man_[1] discusses the odds and convincingly argues for a large number of habitable planets, but it's pretty out of date. His ACCRETE simulation is great at producing solar systems that resemble our own, but it is not well-respected by astronomers. The time I spoke with an astronomer about this, he said that current simulations have trouble getting from rocks to planets. ACCRETE is a pretty crude simulation; to Dole and his coauthors the interesting thing about it was that such a crude instrument could produce such legitimate-looking results.

One of ACCRETE's problems is that there are a number of parameters, and if you change them slightly you get very different solar systems. I would think a good simulation would have a small number of these parameters but they usually give you enough rope to hang yourself. In any event, to choose the right settings you must have either physics dictating them or accurate numbers from samples, and we still have a very small set of examples.

As an aside, the sheer volume of stars makes a strong argument that even if the odds are vanishingly small, there ought to still be a mind-numbing of habitable planets.

[1]: https://www.amazon.com/Habitable-Planets-Man-Stephen-Dole/dp...

> As an aside, the sheer volume of stars makes a strong argument that even if the odds are vanishingly small, there ought to still be a mind-numbing of habitable planets.

In a Universe that's extremely large, or potentially infinite, the main question is not the number of habitable planets. It's the density, or the mean distance between neighbors - in space as well as in time.

The issue here is not so much the presence of a planet with the right mix of elements and environmental properties. The size of the universe indicates this is likely, just as you said. The issue is abiogenesis. We have absolutely zero data on how often abiogenesis occurs in nature. We don't actively observe it occurring on earth and have yet to encounter it elsewhere even once. Therefore, no matter how common a "habitable" planet is, without living things to terraform its surface and atmosphere, it's not actually habitable. It needs ecosystems to sustain human beings. And we have no way of knowing how likely or unlikely it is for life to exist.

> Direct imaging of planets is not feasible at an intergalactic scale;

Do you mean transgalactic rather than intergalactic? (It's pretty obvious we're not going to be imaging exoplanets in Andromeda anytime soon...)

The book was published around the same time as the study. I'll be keeping an eye our for newer sources. There's been a lot of interesting discoveries in recent years.

The book was reprinted in 2007, but was originally authored in the 60s.

There was a Scientific America article that discussed something similar but the major point of the article was that there will more likely be habitable planets in the future. That is the universe is still in its early days of habitable planets/zones.

I read the article casually before going to bed and can't recall that many of the details.

I'll try to find the article and append a link (I believe the article even referenced the same study).

EDIT I guess it was the smithsonian but I swear it was SA: http://www.smithsonianmag.com/science-nature/why-universe-be...

The whole "colonize planets" won't pan anyway. By the time it is viable a O'Neill cylinder will be a better place to live and we can put them in ideal orbits.

Right now, this is more about searching for other life.

Considering that all our eggs are in one basket, right now, any topic of discussion in the vein of colonization is a good one, as long as it keeps people thinking in that direction, I'd say.

In the short term, their are still the intra-solar-system planets and satellites, and those are something we should push onto quickly. Once we get to those, permanent artificial platforms will arrive all the faster, if at all.

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